P0091 on the 6.7L Power Stroke is a fuel rail pressure regulator control circuit low fault. The 6.7L uses a Bosch CP4.2 high pressure pump with a volume control valve (also called the inlet metering valve) that regulates how much fuel is drawn into the pump. When the ECM commands this valve and sees the control circuit voltage low, it stores P0091 and can drop the truck into a reduced power state.
What P0091 means
The volume control valve meters fuel entering the CP4.2 pump, and the ECM uses it along with the pressure control valve to hold commanded rail pressure. P0091 specifically flags the control side of that circuit reading low, which usually means a short to ground, an open circuit that is pulling the signal low, or a valve that is not responding correctly.
Common causes
- Failed volume control (inlet metering) valve on the CP4.2 pump
- Wiring shorted to ground or chafed against the engine
- Corroded or loose connector at the injection pump
- ECM driver circuit fault (less common)
- Damage from a prior CP4.2 pump failure event
Symptoms
- Check engine light with reduced engine power
- Hard start or long crank time
- Rail pressure that does not track the commanded value
- Rough idle or stalling
- Related rail pressure or metering codes stored together
How to diagnose it
Pull all codes first, because P0091 alongside metallic fuel or multiple injector codes can signal a CP4.2 pump failure that has contaminated the system. Inspect the connector and harness at the pump for chafing and corrosion. Measure the valve resistance and compare to specification, and check the two control wires for shorts to ground and for continuity back to the ECM. Monitor desired versus actual rail pressure with a scan tool while the engine cranks or idles.
How to fix it
Repair shorted or broken wiring and clean corroded connectors, which resolves many P0091 cases. If the volume control valve fails its resistance test or the circuit checks good but rail pressure still will not regulate, replace the valve. If you find metal debris in the fuel system, the entire high pressure side including the CP4.2 pump, injectors, rails, and lines may need replacement to prevent repeat failures. Clear the code and road test after repair. For CP4.2 pumps, volume control valves, or a full disaster-prevention kit, contact sales@coredieselparts.com.
Frequently asked questions
Does P0091 mean my CP4.2 pump failed?
Not always. P0091 is a circuit code first, so check wiring and the metering valve. But if it appears with injector faults or you find metal in the fuel, suspect a CP4.2 failure and inspect the whole system.
What is the difference between P0091 and P0092?
P0091 is the control circuit reading low and P0092 is the circuit reading high. Both point at the same volume control valve circuit but from opposite electrical directions.
Can I keep driving with P0091?
Avoid it. The truck may be in reduced power, and if a pump failure caused the code, continued driving can spread metal contamination and increase the repair cost dramatically.
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